. >> reporter: hitting the fast food drive-in for science. >> can i get five soft tacos? can i get two sausage burritos? can i get a number four? >> reporter: over and over and over again, it has been dave giocolo's life the past three months or so. as has been eating the food. a precisely measured 1,000 calories a day. a fast food daily only. how many calories? >> 770. >> reporter: why? this man is paying them to. dr. samuel klein is a researcher at washington university in st. louis. trying to understand why fast food leads to weight gain and diseases like diabetes and hypertension. at some point, though, research on rats alone is not enough. somebody has to eat the food? >> ultimately, it has to come to people. >> reporter: and so, a radio ad, that offered a cash incentive, up to $3,500, depending on how long it would take. >> once i got into work, i called right away. >> reporter: so did nurse, dawn freeman. >> it probably took a month, to just get -- >> reporter: to get approved? >> to get approved. >> reporter: so, they ate every day, from one of these five restaura